Inhibiting propagation of surface cracks in a MEMS Device

ABSTRACT

A microelectromechanical systems (MEMS) device ( 58 ) includes a structural layer ( 78 ) having a top surface ( 86 ). The top surface ( 86 ) includes surface regions ( 92, 94 ) that are generally parallel to one another but are offset relative to one another such that a stress concentration location ( 90 ) is formed between them. Laterally propagating shallow surface cracks ( 44 ) have a tendency to form in the structural layer ( 78 ), especially near the joints ( 102 ) between the surface regions ( 92, 94 ). A method ( 50 ) entails fabricating ( 52 ) the MEMS device ( 58 ) and forming ( 54 ) trenches ( 56 ) in the top surface ( 86 ) of the structural layer ( 78 ) of the MEMS device ( 58 ). The trenches ( 56 ) act as a crack inhibition feature to largely prevent the formation of deep cracks in structural layer ( 78 ) which might otherwise result in MEMS device failure.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to microelectromechanicalsystems (MEMS) devices. More specifically, the present invention relatesto methodology for inhibiting the propagation of cracks in the surfaceof a MEMS device.

BACKGROUND OF THE INVENTION

Microelectromechanical Systems (MEMS) devices are widely used inapplications such as automotive, inertial guidance systems, householdappliances, protection systems for a variety of devices, cellularcommunication devices, and many other industrial, scientific, andengineering systems. Some MEMS devices may be used to sense a physicalcondition such as acceleration, pressure, angular rotation, ortemperature, and to provide an electrical signal representative of thesensed physical condition to the applications and/or systems employingthe MEMS sensors. Other MEMS devices may be utilized as actuators,switches, pumps, and so forth.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the Figures (not necessarily drawn to scale), whereinlike reference numbers refer to similar items throughout the Figures,and:

FIG. 1 shows a side view of a portion of a MEMS device;

FIG. 2 shows an enlarged top view of a portion of the MEMS device;

FIG. 3 shows a flowchart of a fabrication process for fabricating a MEMSdevice incorporating features for inhibiting the propagation of surfacecracks in the MEMS device in accordance with an embodiment;

FIG. 4 shows a partial cross-sectional view of a structure at an initialstage of manufacture for producing a MEMS device having featuresincorporated therein for inhibiting the propagation of surface cracks;

FIG. 5 shows a partial cross-sectional view of the structure of FIG. 4at a subsequent stage of processing;

FIG. 6 shows a partial cross-sectional view of the structure of FIG. 5at a subsequent stage of processing;

FIG. 7 shows a partial cross-sectional view of the structure of FIG. 6at a subsequent stage of processing;

FIG. 8 shows a partial cross-sectional view of the structure of FIG. 7at a subsequent stage of processing;

FIG. 9 shows a partial cross-sectional view of the structure of FIG. 8at a subsequent stage of processing;

FIG. 10 shows a partial cross-sectional view of a MEMS device fabricatedfrom the structure of FIGS. 4-9 having features formed therein thatinhibit the propagation of cracks in the surface of the MEMS device; and

FIG. 11 shows an enlarged partial top view of the MEMS device of FIG.10.

DETAILED DESCRIPTION

Numerous MEMS devices have been developed which use polycrystallinesilicon (polysilicon) as a primary structural material. It has beenobserved, however, that polysilicon can crack during MEMS devicefabrication, as well as under potentially severe mechanical andenvironmental loading conditions. The cracks tend to form at thepolysilicon surface and propagate a relatively long distance laterallyacross the surface of the polysilicon before propagating deeply into thepolysilicon material, resulting in MEMS device failure.

Referring to FIGS. 1 and 2, FIG. 1 shows a side view of a portion of anexemplary MEMS device 20, and FIG. 2 shows an enlarged top view of aportion of MEMS device 20. In this example, MEMS device 20 may be formedby depositing, patterning, and etching a series of layers onto anunderlying substrate 22. For example, MEMS device 20 may includeelectrodes 24 formed on substrate 22, a layer of nitride film 26, and apolysilicon structural layer 28, each of which are suitably patternedand etched to yield MEMS device 20. In some devices, polysiliconstructural layer 28 may be suspended above, i.e., separated by an air orvacuum gap from, the underlying electrodes 24 and/or nitride film 26. Insuch a design, polysilicon structural layer 28 interconnects to theunderlying material layers at anchor regions 30 or by a system ofanchors and springs (not shown). In other devices, additional materiallayers may be formed on top of polysilicon structural layer 28.

Due to the conventional surface micromachining processes, of deposition,patterning, and etching, a top surface 32 of polysilicon structurallayer 28 may not lie in one plane. That is, top surface 32 may not beflat or planar. For example, a step-down 34 may occur at theintersection of a surface region 36 and a surface region 38 of topsurface 32 near anchor regions 30. Step-down 34 represents aninconsistency, i.e., a change in planarity of top surface 32. As such,surface region 36 may generally lie in a plane 40 and surface region 38may generally lie generally in a plane 42 that is parallel to, but isoffset from, plane 40.

It has been observed that surface cracks 44 (one shown in FIG. 2) tendto initiate at top surface 32 of polysilicon structural layer 28 nearanchor regions 30. Unfortunately, these surface cracks 44 can propagatetoward the suspended structures within polysilicon structural layer 28of MEMS device 20. Surface cracks 44 may tend to initiate at anchorregions 30 because the nonplanar surface topology of top surface 32creates a tensile stress concentration point. In general, surface cracks44 can start when the tensile stress exceeds the material strength. Inaddition, the grain structure of polysilicon structural layer 28 may notbe cohesive around the anchor regions 30. The merging of two differentgrain orientations of polysilicon can result in a seam at the anchoredge, i.e., at step-downs 34, that may be weaker than the rest ofpolysilicon structural layer 28.

Unfortunately, polysilicon cracking can result in a yield loss. In someexamples, the yield loss can run around approximately five percent.Tests may be run on MEMS devices to detect cracks in the polysilicon.While testing might be viewed as a practical solution to reliabilityassurance of MEMS devices, the cost for developing effective andreliable testing remains high, which ultimately increases the cost forthe MEMS devices. Furthermore, the tests may not be one hundred percenteffective at detecting polysilicon cracks. Thus, some MEMS devices canstill get shipped into the field, resulting in customer returns of thecracked units.

Embodiments described herein entail methodology for fabricating a MEMSdevice by incorporating features for inhibiting the propagation ofsurface cracks in the MEMS device, and a MEMS device that includesfeatures that inhibit the propagation of cracks in its polysiliconsurface. Such methodology and MEMS device structure can result inreduced tests costs, MEMS device yield improvement, and improved qualityand reliability of the MEMS devices in the field.

FIG. 3 shows a flowchart of a fabrication process 50 for fabricating aMEMS device incorporating features for inhibiting the propagation ofsurface cracks in the MEMS device in accordance with an embodiment. Thevarious method steps depicted in FIG. 3 will be described in more detailbelow in connection with FIGS. 4-11. Accordingly, the followingdiscussion of fabrication process 50 should be considered a summary ofthe method, and the various embodiment details discussed below inconnection with FIGS. 4-11 apply to the discussion of the method stepsof FIG. 3.

In general, the method begins, at a task 52, at which a MEMS device isfabricated. The MEMS device may be MEMS device 20 mentioned above, orany other MEMS device design in which surface cracking of a polysiliconstructural layer may occur. Next, at a task 54, trenches are formed inthe surface of the MEMS device. FIGS. 4-9 generally show schematiccross-sectional views illustrating a number of MEMS operationsencompassed within the summary task 52 of fabrication process 50, andFIGS. 10-11 generally show schematic cross-sectional views illustratingtrenches 56 (see FIGS. 10-11) formed in a MEMS device 58 (see FIGS.10-11) in accordance with the operations encompassed within the summarytask 54 of fabrication process 50.

The trenches may be formed by implementing a suitable manufacturingprocess, such as by etching, although the particular trench formationmanufacturing process is not a limitation. The trenches are formedanywhere in the surface of the polysilicon structural layer at stressconcentration points at which cracks may likely initiate in the future,such as at the location of step-downs 34 (FIG. 1) or step-ups. Thetrenches may allow small, i.e., relatively short, surface cracks toform. However, suitably arranged trenches largely prevent the smallsurface cracks from propagating laterally far enough to cause MEMSdevice failure. Following the formation of the trenches at task 54, MEMSdevice fabrication process 50 ends. Of course, there may be additionaloperations prior to the completion of MEMS device fabrication process 50that are omitted herein for brevity.

Referring now to FIG. 4, FIG. 4 shows a partial cross-sectional view ofa structure 60 at an initial stage 62 of manufacture for producing aMEMS device, e.g., MEMS device 58 (FIG. 10), having featuresincorporated therein for inhibiting the propagation of surface cracks 44(FIG. 2). Different shading and/or hatching is utilized in theillustrations to distinguish the different material layers and/or thedifferent elements within the structural layers.

In general, at initial stage 62, a polysilicon layer 64 is deposited ona substrate 66 and patterned using, for example, a photolithographicprocess, and etched using, for example, reactive ion etching (RIE). Ahigh conductivity may be desired for polysilicon layer 64 in someembodiments. Hence, polysilicon layer 64 may be doped over the entiresurface area, or may otherwise be made highly conductive to yield, forexample, electrodes (such as electrodes 24 shown in FIG. 1). Thoseskilled in the art will readily recognize that prior to initial stage 62at which polysilicon layer 60 is formed over substrate 66, varioussurface preparation operations may be performed that are omitted hereinfor brevity.

FIG. 5 shows a partial cross-sectional view of structure 60 at asubsequent stage 68 of processing. At stage 68, nitride 70 is depositedover polysilicon layer 64 as well as any exposed portions of substrate66. Nitride 70 may be patterned using, for example, a photolithographicprocess, and etched using, for example, RIE, to produce a patternednitride layer. Nitride 70 insulates various regions of polysilicon layer64 from one another.

FIG. 6 shows a partial cross-sectional view of structure 60 at asubsequent stage 72 of processing. At stage 72, a sacrificial oxide 74is deposited over substrate 66, polysilicon layer 64, and nitride 70.Sacrificial oxide 74 may then be patterned using, for example, aphotolithographic process, and etched using, for example, an oxide wetetch process.

FIG. 7 shows a partial cross-sectional view of structure 60 at asubsequent stage 76 of processing. At stage 76, thick polysilicondeposition is performed. As shown, a polysilicon structural layer 78 isformed overlying the various structures previously built up on substrate66. Polysilicon structural layer 78, including any openings extendingthrough layer 78, may be suitably formed using various processes forthick film deposition, patterning, and etching.

FIG. 8 shows a partial cross-sectional view of structure 60 at asubsequent stage 80 of processing. At stage 80, etching is performed toremove sacrificial layer 74, which was illustrated in FIGS. 6 and 7, butis no longer visible in FIG. 8. Following removal of sacrificial layer74, the microstructures of MEMS device 58 (FIG. 10) are released and arespaced apart from the underlying substrate 66, polysilicon layer 64, andnitride 70. Accordingly, any movable structures (not expressly labeled)of MEMS device 58 are now movably suspended in accordance with aparticular design of MEMS device 58.

FIG. 9 shows a partial cross-sectional view of structure 60 at asubsequent stage 82 of processing. At stage 82, a sealing material 84,e.g., a packaging material, may be deposited over portions of structure60 to seal otherwise protect at least portions of structure 60 from theexternal environment. However, remaining portions of structure 60 maynot be encapsulated in sealing material 84 when the movablemicrostructures are to be exposed to an external environment in order todetect an external stimulus (e.g., pressure).

FIGS. 4-9 are discussed in connection with surface micromachiningoperations implemented to form a MEMS device. It is to be understoodthat certain operations depicted in FIGS. 4-9 may be performed inparallel with each other or with performing other processes, or may beomitted in accordance with particular MEMS device fabricationmethodologies. Furthermore, the particular deposition, patterning, andetching techniques mentioned herein are not a limitation. Rather, anysuitable technique for deposition, patterning, etching, and so forth maybe implemented in accordance with alternative embodiments. In addition,it is to be understood that the particular ordering of the operationsdepicted in connection with FIGS. 4-9 may be modified, while achievingsubstantially the same result.

Referring now to FIGS. 10-11, FIG. 10 shows a partial cross-sectionalview of MEMS device 58 fabricated from structure 60 having featuresformed therein that inhibit the propagation of cracks in a top surface86 of MEMS device 58, and FIG. 11 shows an enlarged partial top view ofMEMS device 58. In accordance with block 54 (FIG. 3) of fabricationprocess 50 (FIG. 3), trenches 56 are formed in a top surface 86 of MEMSdevice 58. In FIG. 10, trenches 56 are represented in dashed line formextending into top surface 86. In FIG. 11, trenches 56 are illustratedusing rightwardly and upwardly wide hatching to distinguish them fromthe surrounding top surface 86 of polysilicon structural layer 78.

Due to the surface micromachining processes, of deposition, patterning,and etching as shown above, top surface 86 of polysilicon structurallayer 78 may not lie in one plane. That is, top surface 86 may not beflat or planar. In this example, a step-down 90 may occur at theintersection of a surface region 92 and a surface region 94 of topsurface 86 near anchor regions 96 (one being visible in FIG. 10). Assuch, surface region 92 lies generally in a plane 98 and surface region94 lies generally in a plane 100 that is parallel to, but is offsetfrom, plane 98. In the illustrated embodiment, surface region 94 extendsupwardly from surface region 92. The intersection of surface regions 92and 94 is a tensile stress concentration location where polysiliconstructural layer 78 may crack during fabrication or in the future. Ofcourse, other step-down regions and/or other tensile stressconcentration location may be present in top surface 86.

Trenches 56 are formed in top surface 86 of polysilicon structural layer78. In particular, trenches 56 are formed across one or morelongitudinal joints 102 between surface regions 92 and 94, and extendinto each of surface regions 92 and 94. Trenches 56 may be formed byetching, saw cutting, or any other suitable technique. In an embodiment,a length 104 of each of trenches 56 is oriented approximatelyperpendicular to the longitudinal joint 102 across which it spans. Thus,trenches 56 spanning the same one of longitudinal joints 102 areparallel to one another. In some embodiments, length 104 of each oftrenches 56 is in a range of three to seven microns, and a width 106 ofeach of trenches 56 is in a range of three to five microns into topsurface 86. A spacing 108 between adjacent trenches 56 may also be in arange of three to five microns. In an embodiment, width 106 and spacing108 are as small as can be manufactured.

In general, shallow surface cracks 44 (FIG. 2) have been observed to beless than 0.25 microns deep. Accordingly, trenches 56 are formed toextend into top surface 86 by a depth 110 in a range of 0.25-0.75microns. In this example embodiment, trenches 56 may extend into surfaceregion 92 by approximately 0.5 microns and trenches 56 may extend intosurface region 94 by 0.5 microns plus the height of surface region 94above surface region 92 (e.g., 0.25 microns). In some embodiments, depth110 can be any suitable amount such that a bottom surface 112 of trench56 extending across longitudinal joint 102 and into surface regions 92and 94 may be generally flat, i.e., is without step-downs or step-ups.In other embodiments, bottom surface 112 of trench 56 may be uneven as aresult of a particular etch process used, e.g., an isotropic etchprocess.

In some embodiments, one of longitudinal joints 102 is continuous withanother one of longitudinal joints 102, but they are oriented out ofalignment with one another, i.e., nonparallel relative to one another.Such a configuration can occur at certain corners 114 of surface regions92 and 94 of top surface 86. Accordingly, a subset 116 of trenches 56may extend across one of longitudinal joints 102, e.g., a longitudinaljoint 102A, oriented perpendicular to that longitudinal joint 102A, andanother subset 118 of trenches 56 may extend across the otherlongitudinal joint 102, oriented perpendicular to that longitudinaljoint 102. As such, trenches 56 extending across longitudinal joints 102that are out of alignment (nonparallel) relative to one another willlikewise be out of alignment (nonparallel) relative to one another. Insome embodiments, these nonparallel trenches 56 in close proximity withone another may intersect to form a contiguous, i.e., single, extendedtrench 120 having a portion that is perpendicular to the longitudinaljoint 102 that it spans and having another portion that is perpendicularto the longitudinal joint 102 that it spans.

The multiple trenches 56 are formed anywhere in top surface 86 ofpolysilicon structural layer 78 where surface cracks 44 (FIG. 2) may belikely to form in top surface 86 of polysilicon structural layer 78. Insome embodiments, the surface of trenches 56 may be filled with apackaging material, i.e., an encapsulant. Nevertheless, when trenches 56are located near anchor regions 96, small (relatively short), shallowsurface cracks 44 may be allowed to form. However, the presence oftrenches 56 prohibits the short surface cracks 44 from propagatinglaterally where they could go deep enough to breach the suspendedstructures and cause MEMS device failure. Thus, trenches 56 can limitthe length of the propagation path of shallow surface cracks 44.

Embodiments described herein entail methodology for fabricating a MEMSdevice by incorporating trench features that can inhibit the propagationof surface cracks in the MEMS device, and a MEMS device that includesthe trench features. Although particular trench configurations aredescribed above, MEMS devices may include trench features having othershapes, depths, orientations, locations, and so forth. These and othervariations are intended to be included within the scope of the inventivesubject matter. Such methodology and MEMS device having trenches formedtherein can result in reduced tests costs, MEMS device yieldimprovement, and improved quality and reliability of the MEMS devices inthe field.

While the principles of the inventive subject matter have beenillustrated and described above in connection with specific devices andmethods, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinventive subject matter. The foregoing description of specificembodiments reveals the general nature of the subject mattersufficiently so that others can, by applying current knowledge, readilymodify and/or adapt it for various applications without departing fromthe general concept. Therefore, such adaptations and modifications arewithin the meaning and range of equivalents of the disclosedembodiments. The inventive subject matter embraces all suchalternatives, modifications, equivalents, and variations as fall withinthe spirit and scope of the appended claims.

What is claimed is:
 1. A method comprising: fabricating amicroelectromechanical systems (MEMS) device that includes a structurallayer having a surface, said surface including a first surface regionand a second surface region adjacent to said first surface region, saidfirst surface region lying in a first plane that is offset from a secondplane of said second surface region, said second plane beingsubstantially parallel to said first plane, said surface including atleast one stress concentration location, said at least one stressconcentration location including a longitudinal joint between said firstand second surface regions; and forming at least one trench in saidsurface of said structural layer across said stress concentrationlocation such that said at least one trench is formed in said surface ofsaid structural layer across said longitudinal joint.
 2. A method asclaimed in claim 1 wherein said at least one trench extends into each ofsaid first and second surface regions.
 3. A method as claimed in claim 1wherein said at least one trench exhibits a length and a width, saidlength being greater than said width, and said length of said at leastone trench is oriented approximately perpendicular to said longitudinaljoint.
 4. A method as claimed in claim 1 wherein said longitudinal jointis a first longitudinal joint, said MEMS device includes a secondlongitudinal joint between said first and second surface regions, saidsecond longitudinal joint being contiguous with said first longitudinaljoint, said second longitudinal joint being in nonparallel alignmentwith said first longitudinal joint section, and said forming operationcomprises: forming a first one of said at least one trench extendingacross said first longitudinal joint; and forming a second one of saidat least one trench extending across said second longitudinal joint,said second trench being in said nonparallel alignment with said firsttrench.
 5. A method as claimed in claim 4 wherein: said first trench isoriented approximately perpendicular to said first longitudinal joint;and said second trench is oriented approximately perpendicular to saidsecond longitudinal joint.
 6. A method as claimed in claim 4 whereinsaid first and second trenches form a contiguous extended trench.
 7. Amethod as claimed in claim 1 wherein said at least one trench extendsinto said surface by a depth, said depth being in a range of 0.25 to0.75 microns.
 8. A method as claimed in claim 1 wherein said at leastone trench exhibits a length and a width, said length being greater thansaid width, and said length being in a range of three to seven microns.9. A method as claimed in claim 1 wherein said at least one trenchexhibits a length and a width, said length being greater than saidwidth, and said width being in a range of three to five microns.
 10. Amethod as claimed in claim 1 wherein said forming operation comprisesforming a plurality of trenches extending across said stressconcentration location.
 11. A method as claimed in claim 10 wherein saidtrenches of said plurality of trenches are oriented approximatelyparallel to one another.
 12. A method as claimed in claim 10 whereinadjacent trenches of said plurality of trenches are separated from oneanother by a spacing, said spacing being in a range of three to fivemicrons.
 13. A method as claimed in claim 1 wherein said structurallayer is a polysilicon layer.
 14. A method comprising: fabricating amicroelectromechanical systems (MEMS) device, said MEMS device includinga polysilicon structural layer, said polysilicon structural layer havinga surface, said surface including a first surface region and a secondsurface region adjacent to said first surface region, said first surfaceregion lying in a first plane that is offset from a second plane of saidsecond surface region, said second plane being substantially parallel tosaid first plane; and forming a plurality of trenches in said surface ofsaid polysilicon structural layer across a longitudinal joint betweensaid first and second surface regions, each of said trenches of saidplurality of trenches extending into said first and second surfaceregions by a depth.
 15. A method as claimed in claim 14 wherein saiddepth is in a range of 0.25 to 0.75 microns.
 16. A method comprising:fabricating a microelectromechanical systems (MEMS) device that includesa structural layer having a surface, said surface including a firstsurface region and a second surface region adjacent to said firstsurface region, said first surface region lying in a first plane that isoffset from a second plane of said second surface region, said secondplane being substantially parallel to said first plane, said surfaceincluding at least one stress concentration location, said at least onestress concentration location including a longitudinal joint betweensaid first and second surface regions; and forming a plurality oftrenches in said surface of said structural layer extending across saidlongitudinal joint, each of said trenches extending into each of saidfirst and second surface regions, wherein adjacent ones of said trenchesextending across said longitudinal joint are oriented approximatelyparallel to one another.
 17. A method as claimed in claim 16 whereineach of said trenches exhibits and length and a width, said length beinggreater than said width, and said length of said each of said trenchesis oriented approximately perpendicular to said longitudinal joint. 18.A method as claimed in claim 16 wherein said longitudinal joint is afirst longitudinal joint, said MEMS device includes a secondlongitudinal joint between said first and second surface regions, saidsecond longitudinal joint being contiguous with said first longitudinaljoint, said second longitudinal joint being in nonparallel alignmentwith said first longitudinal joint section, and said forming operationcomprises: forming a first one of said plurality of trenches extendingacross said first longitudinal joint; and forming a second one of saidplurality of trenches extending across said second longitudinal joint,said second trench being in said nonparallel alignment with said firsttrench, and said first and second trenches forming a contiguous extendedtrench.
 19. A method as claimed in claim 18 wherein: said first trenchis oriented approximately perpendicular to said first longitudinaljoint; and said second trench is oriented approximately perpendicular tosaid second longitudinal joint.